The invention relates to a method of and means for preventing or delaying the formation of undesired phosphine levels in a space or environment, the phosphine being derived from phosphine liberating compositions, in particular comprising hydrolysable metal phosphides, by delaying the onset of phosphine generation and/or by removal of phosphine already generated.
Pest control agents in solid form are known which when exposed to air slowly evolve gaseous components including phosphine. Such pest control agents are, for example adapted to generate hydrogen phosphide from hydrolysable alkaline earth and/or earth metal phosphides, in particular from aluminium phosphide, calcium phosphide and magnesium phosphide, when acted upon by the moisture content of air or stored commodities. Such pest control agents are employed for example for combating pests such as beetles, bugs, worms, cockroaches or other insects and their eggs and larvae but also rodents, e.g. mice and rats, e.g. in stores, mills, shipholds, railway carriages, trucks, containers, storage sheds, plastic lined earth silos and dumps in sheds or under canvas or permanent silos for grain, legumes, nuts, cocoa beans, tobacco or other foodstuffs or luxury goods, animal feeds and other processed or unprocessed commodities and various environments whether used for storage or containing such commodities or not.
Such phosphine releasing pest control agents have in the past 10 to 20 years gained the upper hand to an increasing extent over other fumigating agents such as methyl bromide, ethylene dibromide and ethylene oxide. Reasons for this are on the one hand the favourable properties of hydrogen phosphide; it in no way impairs the quality of most fumigated commodities, rapidly penetrates into the interior, e.g. in the case of grain into the seeds, and there destroys animal organisms including all their development stages. Yet after the fumigation has been completed, the phosphine is dissipated just as rapidly again by aeration. On the other hand the application is relatively simple because the products, e.g. in the form of pressed bodies such as tablets and pellets can either be admixed to the flow of grain or be spread out on the floor of storage halls. However, such pest control agents can also be filled as compositions in powder form into dispensers in the form of sachets of special paper or of suitable non-woven fabrics (fleeces) and can in that form be introduced into the stored commodities. The pest control agent must then release the hydrogen phosphide formed by hydrolysis through the walls of the sachets to the outside. A further development of this method of application for the fumigation process involves the use of a belt comprising a multitude of pockets for the accommodation of the pest control agent either directly or prepacked in separate sachets.
The problem of the formation of undesired phosphine levels as referred to above, arises for example in the following contexts:
(a) in the manufacture, packaging and storage of phosphine liberating compositions, more particularly those based on hydrolysable metal phosphide such as alkaline earth metal phosphides and earth metal phosphides, e.g. magnesium phosphide, calcium phosphide and aluminium phosphide. When such metal phosphides are processed in order to produce marketable products, notably pest control agents and pest control articles, such as powders or granulates filled into the abovementioned dispensers such as gas and moisture-pervious sachets or composite dispensers composed of a plurality of sachets, e.g. those known as bag blankets, or tablets or pellets, the metal phosphide has to pass through a variety of processing steps such as grinding, mixing with other ingredients, filling into the dispenser sachets or other dispenser articles, pressing into pellets or tablets or other bodies with or without special binding agents, mixing with fibres and bonding agents and shaping the mixture into plates or like bodies which in turn are then covered with moisture-pervious covering material such as paper. As a last step the product is sealed into some gastight and moisture-proof container for storage and transport, wherein the product is kept until just before the intended use. During all of those operations it is in practice difficult, if not impossible, to avoid contact of the metal phosphide with moisture, either environmental humidity or moisture contained in the materials with which the metal phosphide is compounded or laminated or covered and packaged. This results in the unavoidable hydrolysis of some of the metal phosphide and the most undesirable loss of the thereby generated phosphine gas to the manufacturing environment. Regardless of the precautions which may be taken in the factory, it is difficult to avoid completely the escape of some of this phosphine gas into the working environment and the exposure of the workers to the inhalation of highly toxic phosphine gas. Although the concentrations released to the working environment are usually very low indeed and well below the levels permitted by factory control legislation, this state of affairs is nevertheless undesirable. Moreover, the immediate environment of manufacture, i.e. the manufacturing equipment, even though well screened to the working environment, must be vented in some form or another. The concentrations of phosphine gas which are thereby released to the outside of the factory and into the environment are small and will not normally constitute a health or environmental hazard, because the concentrations are so low and because phosphine gas does not cumulate in living organism nor in the environment where it decomposes soon into products which are completely harmless in the low concentrations involved. Nevertheless, even those low concentrations released to the environment create an odour nuisance (a smell, similar to that of garlic or onions and which is also well known to the users of industrial acetylene gas which is normally contaminated with traces of phosphine). No satisfactory solution to this problem has yet been found.
(b) The problem of undesired phosphine release and accumulation continues inside the package. This is due to the unavoidable inclusion of traces of moisture in the package derived from the materials employed. This phosphine accumulation can be dangerous when the package is opened, because the sudden contact of the phosphine with the atmosphere may result in auto-ignition, known as flashing. When the product is in skilled hands, the phenomenon of flashing, is usually harmless, but is nevertheless frightening. However, flashing can be a fire hazard in fire-susceptible environments.
According to the prior art, the aforesaid problems have been counteracted
(i) by avoiding as far as possible the inclusion of moisture and contact with moisture in the environment wherein processing and packaging takes place, and the choice of materials such as additive and packaging materials which as far as possible are moisture-free, but can never be moisture-free entirely; PA1 (ii) by including inside the package a means for destroying phosphine, usually in the form of sachets filled with a phosphine destroying composition, including copper oxide and similar heavy metal compositions. This, however, is unsatisfactory, because the phosphine destroying composition is progressively used up, moreover the destruction of phosphine involves moisture-liberating reactions, whereby more moisture becomes available which in turn enters into reaction with the metal phosphide, to release more phosphine. In addition, the use of heavy metal composition is nowadays considered environmentally undesirable.
(c) Finally and very importantly, there is the problem of premature release of phosphine and the buildup of undesirable levels after the metal phosphide composition is removed from the package for actual use, e.g. the distribution of the product in a bulk material or in a storage or transportation space to be fumigated with phosphine. As aforementioned, phosphine gas is extremely toxic, and the exposure of fumigation workers to phosphine gas in appreciable concentrations must be avoided. This can in certain circumstances be very difficult, because of the time delay between the opening of the first package and the introduction of the product into the environment to be fumigated. In the past such operations have sometimes been impossible to carry out without the use of gas masks which are unpleasant to the user and can become defective after prolonged use. Fortunately, in contrast to the use of other gases, it is usually not necessary to wear gas-masks when employing hydrogen phosphide developing pest control agents. Nevertheless, measurable hydrogen phosphide concentrations do arise even during the application stage--in particular when the temperatures and moisture are relatively high.
In the light of ever-increasing environmental consciousness, this fact is giving rise to increasing criticism, because hydrogen phosphide is highly toxic also for humans and higher animals. The fact that the phosphine concentrations in the aforesaid situations, if proper precautions are taken, are well below accepted danger levels is no longer accepted by the public.
The result has been that in some countries or in part of such countries labour organisations are prohibiting the application of hydrogen phosphide developing pest control agents by their members, not the least because, in the past several accidents with fatal consequences had to be contended with, due to faulty user practices.
In order to mitigate the high moisture sensitivity of the abovementioned preparations during handling, packaging and application, attempts had previously been made (DE-GM 1829597) to coat the phosphide particles or the entire tablets with hard paraffin, natural or synthetic resins, waxes or silicones. From those unsuccessful attempts a further proposal arose (U.S. Pat. No. 3,132,067) to completely envelop the individual phosphide particles with a coating, in particular of hard paraffin wax. In order to permit the required access to the phoshide particles of ambient moisture when using the preparation, it was considered necessary to incorporate a bursting agent such as ammonium carbamate. These coatings of moisture-impervious material, and including ammonium carbamate, although intended to delay the onset of phosphine generation, did not achieve this in practice. If the mass loss of the tablets due to the loss of carbamate was taken into account it was found that these tablets in fact commenced releasing substantial phosphine within minutes of exposure to humidity and continued doing so for several days, albeit at a reduced rate and with a reduced risk of autoignition.
More recent attempts to improve the techniques of coating tablets in order to produce a controlled delay of the onset of phosphine release are still in an early development stage. These techniques are sophisticated and cannot be applied to many products such as plates, powders and sachets, bag blankets and such like. There thus exists a previously unsatisfied need to overcome or mitigate the aforesaid problems.